most cars only have mini jacks...so if you pop the front tyre, do you really want to have to jack up the front, take the bad tyre off and put on the spacesaver....then jack up the back, take off the good tyre and replace with the bad tyre...jack up the front swap the spacesaver for the good tyre from the rear, then jack up the back once more and put on the spacesaver?
Also if your on a spacesaver you shouldnt be going hard at all so it shouldnt really have anymore wear being on the front as apposed to the rear...all tyres spin